(1) Field of the Invention
The invention relates to a a thin film transistor for use in conjunction with an active matrix liquid crystal display or the like, and more particularly to a method of manufacturing a thin film transistor in which a light-absorbing layer is added so that a laser can be safely used to irradiate and recrystallize the conductive layer of the transistor. The conductive layer is changed from amorphous silicon into polysilicon after the laser anneal.
(2) Background Art
In order to reduce both the cost and size of active matrix liquid crystal displays (LCD), the LCD driver circuits need to be integrated onto the glass substrate on which the LCD in formed. Polysilicon (poly-si) thin film transistor (TFT) driver circuits manufactured using low-pressure chemical vapor deposition (LPCVD) have been successfully applied in small-size liquid crystal displays. However, LPCVD is a high-temperature process, usually in excess of 600.degree. C., and requires the use of a high-cost quartz substrate. Since amorphous silicon (a-Si) thin film transistors can be grown on low-cost glass substrates at lower temperatures, usually below 300.degree. C., through plasma-enhanced chemical vapor deposition, and since they have a low off current and a high on current/off current ratio, a-si TFT's have been widely used as switching elements in large-area liquid crystal displays currently in mass production. However, it is difficult to use a-Si thin film transistors in the LCD driver circuits, due to a-Si's low mobility, which is less than 1 cm.sup.2 /V.multidot.sec.
As an alternative to using LPCVD, poly-Si TFTs can also be manufactured through the use of low-temperature laser recrystallization. However, due to their high off current and uniformity problems, poly-Si transistors are not sufficient for use as a pixel-switching element. In the interests of the production of low-cost active matrix liquid crystal displays, it would be advantageous to combine the merits of a-Si thin film transistors and laser-annealed poly-Si thin film transistors. One method is to use an a-Si thin film transistor as the switching element and laser-annealed poly-Si thin film transistors in the peripheral driver circuit.
A-Si TFTs are often manufactured using the inverted staggered structure, as shown in FIG. 1. There is shown a gate metal 1, a gate dielectric 2, intrinsic a-Si 3, an n+ a-Si layer 4, and source and drain electrodes 5.
FIG. 2 shows one of the alternatives to the common inverted staggered structure a-Si thin film transistor design, modified to be able to be used in the LCD peripheral driver circuit. The gate electrode 10 and the gate dielectric layer 9 are formed under the a-Si layer 8. Above the a-Si layer 8, a layer of silicon nitride (SiNx) 6 is commonly used as an etching stopper to prevent damage to a-Si layer 8 during etching of the a-si layer 6B to form the TFT source and drain. Source/drain metal contacts 6A are formed over a-si layer 6B. An insulating layer 7A formed of silicon nitride is formed over the source/drain metal contacts 6A.
Since a-Si is a photo-sensitive material, a layer of black resin 7, used to absorb incident light, is formed over layer 7A. However, with the high intensities of incident light experienced when this type of transistor is used to drive an image sensor, this layer cannot completely prevent the transmission of light through to the semiconductor layer 8. Because the semiconductor layer 8 is made of a-Si, a photo-sensitive material, the photo-leakage current of this layer will be increased when it is irradiated. The resultant photo-leakage current causes the transistor to become unstable.
U.S. Pat. No. 5,371,398 to Y. Nichihara discloses a method of manufacturing a thin film transistor, with the resultant structure shown in FIG. 3, comprising a substrate 11, a gate electrode 12, a gate insulating film 13, a first a-Si layer 14, a protecting film 15, a second a-Si layer 16, a diffusion preventing layer 17, an insulating layer 18 having apertures 19 above the diffusion-preventing layer 17, source and drain electrodes 20 connected to the diffusion preventing layer 17 through the apertures 19, and a light-intercepting layer 21. The structure substitutes a metal layer 21 for the black resin 7 of the FIG. 2 structure, in order to reduce the incidence of light on the semiconductor layer 14. However, the a-Si semiconductor layer is still susceptible to photo-leakage current when the transistor is used for an image sensor. In addition, as previously mentioned, the low mobility of a-Si causes difficulty in use in an active matrix LCD driver circuit.